Coffee with cream cools about 20% slower than black coffee.

For three reasons:
#1 Black coffee is darker.
Dark colors emit heat faster than light colors.
#2 Stefan-Boltzmann says so.
The Stefan-Boltzmann law says that hotter surfaces radiate heat faster—specifically, the power of emission is proportional to the temperature (in kelvin) raised to the fourth power.
#3 Viscosity versus evaporation.
This is the clincher. Adding cream thickens the coffee (adds viscosity), so it evaporates slower.

Image credit: Diet Doctor
Source: modernistcuisine.com/201…